Enh: make the script files ends with the .py extension and make the link...

... to it without extension.

This helps when working with IDE which recognize the .py files.

Also, previously, having the link with the .py extension was quite useless in fact.

steps done:
```
for each plugin
do
    cd $plugindir
    git rm $link_with_py_extension
    git mv ${plugin_script} ${plugin_script}.py
    ln -s ${plugin_script}.py ${plugin_script}
    git add ${plugin_script}
    edit debian/install and add ${plugin_script}.py
done
```